1. Field of the Invention
The present invention relates generally to a metal fitting for composite electrical insulators, and more particularly to a metal fitting which is to be fixedly secured to one end portion of a plastic rod of the insulator for firmly and stably clamping the rod.
2. Description of the Related Art
A composite electrical insulator is known, e.g., from U.S. Pat. No. 4,654,478, wherein one end portion of a fiber-reinforced plastic rod applied with an adhesive material is inserted into the bore in a sleeve portion of the metal fitting and the metal fitting is then fixedly secured to the plastic rod. Such a metal fitting serves to clamp the rod and thereby connect the insulator to an electric cable or the like. The metal fitting is usually subjected to caulking, i.e., compressed radially inwardly onto the plastic rod, so as to firmly clamp the rod. That is to say, by compressing the metal fitting radially inwardly with a suitable die, that region of the plastic rod situated opposite to the metal fitting is uniformly clamped to integrally connect the metal fitting with the plastic rod for preventing withdrawal of the plastic rod from the fitting even under a large tensile force.
The composite electrical insulator as known from U.S. Pat. No. 4,654,478 proved to be highly advantageous in that it is light in weight and has a sufficient mechanical strength. However, there may be instances in a normal use condition of the insulator, wherein the plastic rod comes to be withdrawn from the metal fitting. Such withdrawal may be caused by a gradually decreased clamping force originating from the initial caulking, and/or upon application of an excessive tensile force to the insulator. In this connection, an increase in the initial clamping force is limited, e.g., in view of the compressive strength characteristic of the plastic material. Therefore, it is highly desirable to effectively prevent the withdrawal of the plastic rod from the metal fitting for a prolonged period, without increasing the initial clamping force.